Field of the Invention
The disclosure generally relates to hydrocarbon floating, production, storage, and/or offloading platforms. Specifically, the disclosure relates to such floating platforms having keel skirts to change a heave response of such platform while floating in water.
Description of the Related Art
A typical line up of offshore platforms for hydrocarbon production, storage, and/or offloading includes a deep draft spar suitable for heave control in deep waters, a semi-submersible platform, a tension leg platform, and specialized ship-shaped floating structures for production, storage, and offloading known as FPSOs. Each type has advantages that have been used in different parts of the world depending on types of weather and environment, depth of the water, and other factors.
Specifically, ship-shaped FPSOs have been used throughout the oil and gas industry for decades. But their transverse motion (roll motion) is more severe than their longitudinal motion (pitch motion). The hull needs a weather-vanning system involving an expensive turret and swivel system internally or externally in a severe environment condition. In addition, the natural period of vertical motion as well as transverse motion is close to a typical wave spectral peak period in most operational fields. Consequently, application of the ship-shaped FPSOs are suggested for mild environments.
To remove the need for the weather-vanning dependency in the ship-shaped vessel, a known non-ship shaped design for FPSOs is cylindrical. Examples include the cylindrical designs shown in the following patents: U.S. Pat. No. 6,945,736, U.S. Pat. No. 7,086,810, and GB 2,253,813. The hull typically has large diameter, and can accommodate a large volume of oil storage with keeping hull stability at the quayside fabrication, during wet tow, and at the installation location. The location of oil storage tank is close to the mean water level that provides benefits for designing the hull structure and processing unit on the topside. Compared with the ship-shaped FPSO, the circular hull shape also reduces the span of internal pipelines necessary to processing.
To improve the hull motion response in a severe sea state, the designed hull can include skirt pontoon at a keel level, which provides a hydrodynamic added mass and damping. Examples include the skirts on cylindrical non-ship shaped designs shown in the following patents: U.S. Pat. No. 8,511,246, U.S. Pat. No. 8,544,402, and U.S. D476,998. However, the large size of a keel skirt makes difficult the hull fabrication at the quayside and subsequent loading of the topside and equipment to the hull. The quayside fabrication facility is often limited in the outreach of a quayside crane when a desired width of the keel skirt is added to the pontoon of the hull. The maximum hull width including the keel skirt at the keel needs to be reduced during the quayside integration.
Therefore, there remains a need for an improved keel skirt to accommodate the limitations of quayside facilities for fabrication of such a floating platform.